1) Arsenic is a biologically active element and can be metabolized by arsenic ( 3) methyltransferase(AS3MT) into methylated arsenics in most organisms. However the catalytic mechanism of AS3MT is not clear. Thus the mechanism of methylation, especially the factors affecting the methylation of iAs3 , such as reductant, should be studied further. Our group systematically studied the binding sites of inorganic arsenic and methyl donor SAM in human AS3MT and the effects of different factors on the enzyme activity. We used the rapid equilibrium kinetic model to study the reaction sequence of arsenic methylation and initially proposed catalytic mechanism of AS3MT. Arsenic has two sides, which can serve as a drug to treat acute promyelocytic leukemia. Now we are engaged in studying the effects of inorganic arsenic and its metabolites on the leukemia in order to elucidate the mechanism of arsenic in the treatment of leukemia.